size of the septic tank (or a volume equivalent to at least one

day's volume of raw wastewater flow) and receives both the effluent

from the septic tank and a portion of the sand filter effluent.

When the recirculation tank is full, the sand filter effluent by-

passes the tank and is discharged (USEPA, 1980).

     Since nitrified sand filter effluent mixes with septic tank

effluent   in  the   recirculation  tank,   it  is  possible   that

denitrification of the nitrified sand filter effluent can take

place through utilization of the carbon source provided by the

septic tank effluent.

     Recirculating sand filters were not originally designed for

nitrogen   removal  and  thus  little data   on the effectiveness

of the nitrogen removal capabilities of recirculating sand

filters is a available from literature. Two recirculating sand

filters which were installed in Michigan generally showed

nitrogen removal rates of 40 - 60% (Louden et. al., 19    ).

      Another study of a recirculating sand filter was conducted

 in which the recirculation tank was replaced with a tank filled

 with rock. The carbon source was added directly to the rock

 tank. When septic tank effluent was used as a carbon source, an

 average of only 25% denitrification was observed in the rock tank

 with the whole system achieving a total nitrogen removal of 36%.

 The low amount of denitrification was assumed to be the result of
 the low C:N ratio found in the rock tank.   A C:N ration of 0.7:1

 was maintained while past studies generally suggest a C:N ratio

 of 1:1 to 3:1 is needed.   Thus the amount and availability of